1. Field of the Invention
The present invention relates to a process for refining coal-based heavy oils, and more generally to a process for treating coal-based heavy oils so as to produce hydrocarbon products which are suitable for the production of high-grade carbon stocks (such as readily graphitizable needle-shaped coke, isotropic carbon and high-grade activated carbon).
2. The Prior Art
Carbon stocks such as readily graphitizable needle-shaped coke, isotropic carbon and high-grade activated carbon are very useful, for example, in making ultra high power graphitic electrodes. In the past, carbon stocks have been produced from a great variety of raw materials, ranging from petroleum-based heavy oils and coal-based heavy oils to synthetic polymers. Petroleum-based heavy oils and coal-based heavy oils have been the most popular raw materials because of their low cost and high carbonization yields, the coal-based heavy oils in particular displaying especially high carbonization yields.
However, use of both petroleum-based heavy oils and coal-based heavy oils have their drawbacks. For example, since petroleum-based heavy oils from most sources contain sulfur, which is a very undesirable component, the number of sources capable of producing low sulfur-containing petroleum-based heavy oils are very limited. On the other hand, although coal-based heavy oils have very low sulfur contents and, as noted above, provide very high carbonization yields, they contain minute amounts of fine, inactive carbonaceous substances, especially quinoline insolubles which, if not removed, detrimentally affect the graphitizing properties of the coal-based heavy oils. This is especially undesirable when attempting to produce therefrom the so-called high-grade carbon stocks (containing needle-shake coke and isotropic carbon).
Methods have been devised for removing quinoline insolubles from conventional coal-based heavy oils so as to enable these heavy oils to produce high-grade carbon stocks. These methods can in fact be divided into four categories:
(1) methods which involve subjecting the coal-based heavy oils to a thermal treatment, thereby enlarging the particular diameters of the contained quinoline insolubles and facilitating separation thereof (for example, as disclosed by Japanese Patent Publication Nos. 32722/1972 and 4334/1958);
(2) methods which involve mixing the coal-based heavy oils with an organic solvent, thereby inducing precipitation of the contained insolubles (which include quinoline insolubles) and permitting separation thereof (for example, as disclosed by JA-OS No. 102303/1977),
(3) methods which involve mixing the coal-based heavy oils with an aliphatic solvent, stirring the mixture at an elevated temperature, then allowing the stirred mixture to stand and cool, thereby inducing precipitation of the contained insolubles (which include quinoline insolubles) and permitting separation thereof (for example, as disclosed by Japanese Patent Publication No. 26481/1974 and JA-OS No. 98720/1976), and
(4) methods which involve mixing the coal-based heavy oils with an aliphatic solvent and an aromatic solvent, thereby inducing precipitation of the contained insolubles (which include quinoline insolubles) and permitting separation thereof (for example, as disclosed by JA-OS No. 78201/1977).
Unfortunately, these known methods have not been found to be commercially feasible. With respect to the category (1) methods, since the precipitated insolubles therein have extremely small particle diameters, their separation proceeds at a low speed and with poor efficiency, and during separation the utilized filter will invariably become clogged. Further, since the coal-based heavy oil must be centrifuged or filtered at an elevated temperature (to cause it to exhibit a lowered viscosity), the operation entails expensive equipment and high costs of maintenance. With respect to the category (2) methods, since the amount of organic solvent used therein generally amounts to 10 to 100 times that of the coal-based heavy oils, these methods are very costly. Moreover, since the precipitated insolubles created by these methods have extremely small particle diameters, their separation is achieved with a very low efficiency. With respect to the category (3) methods, since the precipitation of insolubles therein generally occurs slowly at normal room temperature, it is necessary that the mixture of coal-based heavy oil and solvent be stirred for several hours at a temperature of not less than 200.degree. C. and left to stand and cool until precipitation of insolubles occurs, followed by the separation of the precipitated insolubles. As a technique for the separation of precipitated insolubles, these methods are not suitable from the standpoint of both practical utility and operational efficiency. Finally, with respect to the category (4) methods, although the precipitated insolubles have relatively large particle diameters and efficient separation is achieved, the amounts of aromatic solvent and aliphatic solvent generally added in amounts is several times that needed to treat petroleum-based heavy oils. Since these added solvents are expensive, the treatment is not economical because of the cost of the devices needed to recycle the solvents. And even if the solvents are not recycled, the operation still proves uneconomical because the yields of carbonization are inevitably lowered. Moreover, since the heavy oil is mixed with two types of solvents, the equipment used must have a proportionately larger size for handling a fixed volume of heavy oil and, therefore, these methods prove less advantageous for commercialization. In addition, the quality control of the solvents, which is necessitated when the solvents are cyclically used, becomes difficult.
As described above, none of the methods so far proposed have proved commercially feasible for the removal of quinoline insolubles from conventional coal-based heavy oils.
It is an object of the present invention to provide a commercially acceptable method for refining coal-based heavy oils, i.e., so as to easily remove quinoline insolubles therefrom. It is furthermore an object of the present invention to provide a commercially acceptable method for treating coal-based heavy oils so as to produce hydrocarbon products suitable for the production of high-quality carbon stocks.